Australia's greatest river is running dry because of a prolonged drought that has exacerbated the problems caused by farmers taking too much water to irrigate unsuitable crops.

Scientists fear that years of below-average rainfall in south-east Australia is turning the once mighty Murray river - known as the Australian Mississippi - from a gushing torrent to a trickling stream.

A build-up of sand and salt is the biggest problem generated by low rainfall that has dramatically changed the nature of the river over the past couple of decades.

"When we first came down here, we had wetlands in front of us," said Richard Owen, whose old shack overlooks the mouth of the Murray as it runs into the Southern ocean. "Now you can just walk up and across the sand. It's just filled up," Mr Owen said.

For the past three years, dredgers have been operating round the clock to keep the river's mouth from silting up. Even temporary respites in the drought - heavy rains last month and earlier in the year - do not seem to make much of an impact on the problem.

A forecast by the Murray-Darling Basin Commission, the organisation set up to manage the waterway, predicts that the total storage capacity of the river system will continue to decline next year, even with average rainfall.

The river is described as a lifeline for the parched region of Australia, feeding water from the tropical north down the Darling river and from the eastern snowfields where the Murray's source lies 1,550 miles from the river's final destination. The basin is also the nation's food bowl, accounting for 41 per cent of the total value of Australia's agricultural sector. That is one of the problems as rice and cotton farmers take huge amounts of water to irrigate crops unsuited to Australia's dry climate.

The Murray-Darling catchment plays a crucial role in supporting Australia's economy and rural life. It covers 1.06 million sq km (0.4m sq miles), or 15 per cent of Australia's landmass, equivalent to an area the size of France and Spain combined.

Throughout the 19th and early 20th century, until rail transport took over, paddle steamers plied the river, transporting wool, wheat and goods from town to town. Mark Twain once likened the river in the 1880s to the Mississippi.

During that period, farmers used the river water to irrigate crops, turning vast areas of arid lands into lush fields. But so much has been taken out and so many areas stripped of trees that river flows are falling and salinity rising as salt is brought to the surface soil with successive flooding and drought.

In an average year, 13,000 million litres of Murray water flows to the sea. But after four years of drought, outflows are now down to an annual 5,000 million litres - a fraction of the flow of comparable rivers such as the Amazon and Yangtze.

The national and state governments are spending about £200m over the next five years in an attempt to boost the flow and stabilise salinity levels. "Doing nothing is not an option," said Wendy Craik, chief executive of the Murray-Darling Basin Commission.

Adelaide, the capital of South Australia state, draws 40 per cent of its water from the river. The government says supplies from the Murray could be unfit to drink within 20 years for the city of about one million.

Salinity projects up and down the river are trying to stop 1,000 tons of salt a day from entering the water system under a plan to stabilise salinity levels. But more than a year on, the Murray Darling Basin Commission is still searching for an extra 260 million litres of water to meet its stated target of returning the river to its previous flow.

Another problem is that the Murray is a slow and lazy river. Rainfall in the upper reaches of the Darling can take three months to make it downstream to Goolwa, so it takes a long time for the river to flush out all the impurities.

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